Bump tornado from 6.4.1 to 6.4.2 in /lambdas/preview

Bumps [tornado](https://github.com/tornadoweb/tornado) from 6.4.1 to 6.4.2.
- [Changelog](https://github.com/tornadoweb/tornado/blob/v6.4.2/docs/releases.rst)
- [Commits](tornadoweb/tornado@v6.4.1...v6.4.2)

---
updated-dependencies:
- dependency-name: tornado
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

lambdas/preview/requirements.txt

@@ -41,7 +41,7 @@ six==1.12.0
 soupsieve==2.4.1
 testpath==0.4.2
 tinycss2==1.2.1
-tornado==6.4.1
+tornado==6.4.2
 traitlets==5.3.0
 typing_extensions==4.5.0
 urllib3==1.26.19